The present invention relates to a process for producing storage-stable, low odor S,S,S-tributylphosphorotrithioate.
S,S,S-tributylphosphorotrithioate is a known cotton defoliant which is produced by reacting phosphorus trichloride with butyl mercaptan and then oxidizing this reaction product. One of the by-products formed when unreacted butyl mercaptan is oxidized is the highly odorous dibutyl disulfide. Dibutyl disulfide is also a by-product formed upon the decomposition of trace amounts of S,S,S-tributylphosphorotrithioite during the oxidation of that material to form the corresponding phosphorotrithioate.
Several approaches have been taken to reduce the levels of dibutyl disulfide formed during the production process and to reduce the residual amounts of dibutyl disulfide present in the product S,S,S-tributylphosphorotrithioate in an effort to reduce the odor problems associated with that material.
In one approach, S,S,S-tributylphosphorotrithioite (the intermediate from which S,S,S-tributylphosphorotrithioate is produced) is produced by reducing the amount of butyl mercaptan used. See, for example, U.S. Pat. No. 2,943,107. In another approach, the phosphorus trichloride and a slight excess of butyl mercaptan are each added simultaneously to a reaction vessel maintained at reaction temperature. See U.S. Ser. No. 07/339,043, filed Apr. 14, 1989.
In each of these processes, however, there is still some unreacted butyl mercaptan present which oxidizes to form the unwanted dibutyl disulfide. This residual disulfide imparts an undesirable odor to the product S,S,S-tributylphosphorotrithioate.
This odor problem is compounded when crude S,S,S-tributylphosphorotrithioate is treated with a caustic material to improve the stability of the final S,S,S-tributylphosphorotrithioate. Unwanted dibutyl disulfide is formed during hydrolysis of crude S,S,S-tributylphosphorotrithioate. If the conditions under which the hydrolysis is carried out are not carefully controlled additional dibutyl disulfide is formed due to the decomposition of the S,S,S-tributylphosphorotrithioate.